Two-Step Optogalvanic Spectroscopy of Neutral Barium: Observation and Interpretation of the Even Levels Above the 6sIonization Limit Between 5.2 and 7 eV

Even parity levels of neutral barium above the 6 s ionization limit have been investigated by using a two-step pulsed laser excitation with optogalvanic detection. From the metastable states, 5 d 6 s , populated by a d.c. discharge in a heat-pipe vapour, we reach the 5 dnl even Rydberg series with J values ranging from 0 to 5, the 6 p 2 1 S 0 state and the low-lying members ( n = 7 and 8) of the 6 pnp series above the 5 d limits. The 5 dnd configurations (18 levels for each n member) are observed from n = 8 up to n = 45 using optogalvanic spectra recorded from five intermediate 5 d 6 p levels: 3 P 1,2 0 , 3 F 3,4 0 and 1 P 1 0 . These eighteen series are observed for the first time. Fine structure is resolved up to n = 25 and has been analyzed using Slater-Condon parametric calculations. In addition, the 5 d 3/2 ns J = 1 and 5 d 5/2 ns J = 3 series are observed up to n = 38 and 42 respectively, as well as sharp resonances interpreted as 5 d 3/2 ng 7/2 J = 2 and 5 d 5/2 ng J = 1, 2 series up to n = 27 and n = 28 respectively. In conclusion, the energies of 636 new levels between 6 s and 5 d limits are given with their term and configuration identifications. Above the 5 d 5/2 limit, broad resonances observed from the 5 d 6 p 1 P 1 0 , 6 s 7 p 1 P 1 0 and 3 P 0,1,2 0 intermediate levels have been identified with levels of the 6 p 7 p and 6 p 8 p configurations using parametric calculations based on the Slater-Condon theory.